User Guide - Digimap

Chapter 2
Introduction to OS MasterMap Topography Layer
The OS MasterMap Topography Layer contains features that represent objects in the physical environment
such as buildings, fields, fences and letter boxes, as well as intangible objects such as county boundaries or
the line of mean high water. There are over 450 million features in the current product. It is available for all of
Great Britain (England, Scotland and Wales). Figure 9 below shows an extract of OS MasterMap
Topography Layer. Geographic features are modelled in such a way as to make it easy to identify buildings,
roads, water and land at a glance. The data has been collected by Ordnance Survey and is based on the
National Grid.
The basic unit of OS MasterMap Topography Layer data is a point, line or polygon feature that represents a
real-world feature. A feature may also have text that goes with it or a symbol. Both are considered a feature
within OS MasterMap. Each feature comes with a set of attribution that provides information, either about the
real-world object it represents or metadata that helps track changes to the feature. The structure of
OS MasterMap, combined with its rich attribution, gives scope to undertake quite complex geographic
analyses and to enhance customers’ data-capture processes. Attribution provides the opportunity to use the
data as far more than a map. Using the product within a GIS, attribution can be used to analyse, sort, query
and visualise the data in many different ways.
Each feature in OS MasterMap Topography Layer has a unique reference called a TOID. The vast majority
of database systems used within organisations rely on the use of unique referencing for the efficient
management of the data stored within them. Each feature also has a version number and a version date as
well as the TOID. As the real-world feature that it represents changes during its life, so the feature within
OS MasterMap will also change in terms of its shape or its attribution. Whilst it remains the same feature –
essentially occupying the same space and having the same function – the TOID will not change, but the
version number will increment and the version date will change. This allows an instance of a feature to be
identified in both space and time.
Figure 9: Extract of OS MasterMap Topography Layer showing all themes
OS MasterMap Topography Layer user guide chapter 2 v1.3 – 07/2007 © Crown copyright Page 16 of 140